Solids exist in either amorphous or crystalline forms. An anhydrous compound is a compound that does not contain water, especially water of crystallization. A hydrate is any compound containing water in the form of H2O molecules, usually, but not always, with a definite content of water by weight. The best-known hydrates are crystalline solids that lose their fundamental structures upon removal of the bound water. In the case of crystalline forms, molecules are positioned in three-dimensional lattice sites. When a compound recrystallizes from a solution or slurry, it may recrystallize with different lattice arrangements, and the different crystalline forms are sometimes referred to as “polymorphs.” The different crystalline forms or polymorphs of a given substance may differ from each other with respect to one or more physical properties (e.g., mechanical strength, compaction behavior, flow properties, particle size, shape, melting point, degree of hydration or salvation, caking tendency, compactability with excipients), chemical properties (e.g., dissolution rate, solubility), and/or biological properties (e.g., bioavailability, pharmacokinetics). The variation in properties among different crystalline forms often makes one crystalline form more desirable or preferred over other forms.
Cidofovir's chemical formula is C8H14N3O6P and its IUPAC name is ({[(S)-1-(4-amino-2-oxo-1,2-dihydropyrimidin-1-yl)-3-hydroxypropan-2-yl]oxy}methyl)phosphonic acid. Cidofovir has also been described as (S)-(1-(4-amino-2-oxopyrimidin-1(2H)-yl)-3-hydroxypropan-2-yloxy)methylphosphonic acid as well as possibly by other chemical names. Its chemical structure is:

Cidofovir was discovered at the Institute of Organic Chemistry and Biochemistry, Prague, and developed by Gilead Sciences. Today, cidofovir is an injectable antiviral medication for the treatment of cytomegalovirus (CMV) retinitis in patients with AIDS. It suppresses CMV replication by selective inhibition of viral DNA polymerase and therefore prevention of viral replication and transcription. It is an acyclic nucleoside phosphonate, and is therefore independent of phosphorylation by viral enzyme, in contrast to, for instance, acyclovir.
Cidofovir is marketed with the brand name Vistide® by Gilead in the United States and by Pfizer in other parts of the world. Vistide® is a sterile, hypertonic aqueous solution for intravenous infusion only. The solution is clear and colorless. It is supplied in clear glass vials, each containing 375 mg of anhydrous cidofovir in 5 mL aqueous solution at a concentration of 75 mg/mL. The formulation is pH-adjusted to 7.4 with sodium hydroxide and/or hydrochloric acid and contains no preservatives. Renal impairment is the major toxicity of Vistide®.
Presently, there are no Orange Book patents listed as having claims which cover Vistide®, although previously U.S. Pat. No. 5,142,051 was listed in the Orange Book for Vistide®. The '051 patent is not directed specifically to cidofovir or its crystalline forms. Instead, it broadly discloses N-phosphonylmethoxyalkyl derivatives of pyrimidine and purine bases.
Because cidofovir has been demonstrated as an effective treatment for patients with AIDS, improved forms of the compound are desired.